In order to improve the property of a work such as hardness, the work is subjected to a hardening processing by high-frequency power. FIG. 24 is an appearance diagram schematically illustrating a general hardening processing. For example, a work 50 to be heated is configured, as shown, to have an extension portion 52 at a bar-like base portion 51 in a coaxial manner. Thus, the bar-like base portion 51 and the extension portion 52 form a substantially L-like cross section. A heating coil 61 is a saddle-type coil. The heating coil 61 is adapted to connect both ends of a semicircular portion 61a with a pair of straight portions 61b, 61b. In order to perform a hardening processing, a retention means which is not shown is firstly allowed to retain the work 50. Then, the heating coil 61 is placed over the work 50 so that the semicircular portion 61a of the heating coil 61 is positioned at the upper face-side of the extension portion 52 and the straight portion 61b of the heating coil 61 is positioned to be parallel with a bar-like base portion 51. In this arrangement, whether the distance between the heating coil 61 and the extension portion 52 is within a predetermined range or not is confirmed. Thereafter, while the work 50 is being rotated, high-frequency power is inputted from a high-frequency inverter 62 to the heating coil 61, thereby subjecting the work to a hardening processing. The reference numeral 63 in the drawing denotes a matching capacitor constituting a parallel resonance circuit with the heating coil 61.
A known induction hardening apparatus used in a hardening processing has an equivalent circuit configuration in which output terminals of a high-frequency inverter have therebetween a matching capacitor and a heating coil that are connected in parallel. In order to assure the hardening quality, it is ideal that the effective power (kW) inputted to the heating coil is preferably actually measured for the control based on this effective power as a reference. The equivalent circuit of a heating coil is represented by a serial connection of inductance and resistance. Furthermore, the work heated by the heating coil functions as a resistance load. A method of monitoring the effective power is a method to measure the phase difference between the voltage (Vcoil) generated at both ends of the heating coil and the coil current (Icoil) flowing in the heating coil to calculate the effective power based on the formula Pkw=cos ΦVcoilIcoil. In the formula, cos Φ represents a power factor (Φ represents a power factor angle).
However, in the case of an induction hardening, many loads have a low power factor and the phase difference between the coil voltage and the coil current as a measurement target is high. Specifically, a parallel circuit of a capacitor and a heating coil has Q of about 10. The power factor may be assumed as a reciprocal number of Q. When Q is 10, the power factor is 0.1 and the power factor angle cp is 84 degrees. Thus, the resultant effective power is small that is calculated by measuring Vcoil and Icoil to integrate these values by an arithmetic circuit. Since this arithmetic circuit is easily influenced by the temperature drift and the fluctuation of a frequency and a phase difference, the current situation is that the effective power of the induction hardening processing cannot be accurately monitored based on the calculation value by the arithmetic circuit.    Patent Document 1: Japanese Published Unexamined Patent Application No. 2002-317224    Patent Document 2: Japanese Published Unexamined Patent Application No. 2000-150126    Patent Document 3: Japanese Published Unexamined Patent Application No. 2003-231923